The primary goals of this research are to characterize morphologically the types of neurons and synaptic contacts present in Hydra littoralis and to develop a model for the functional architecture of a phylogenetically primitive nervous system. The techniques will include conventional transmission electron microscopy, high voltage electron stereomicroscopy and light microscopy of conventionally stained and selectively stained serial thin and thick sections of Hydra tissues and scanning electron stereomicroscopy of external and fractured internal surfaces of critical-point dried specimens. The results of this study may help to elucidate the underlying principles of structure, function, and evolution of the nervous system. The combination of conventional and specialized electron microscopic techniques developed as part of this research on primitive tissues can be directly applied to the study of normal and diseased tissues in humans and other animals.